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1)  ASTRONOMICAL UNIT

1 AU = 149,597,870.700 kilometers

Definition: An Astronomical Unit is the mean distance between the Earth and the

Sun. In 2012, the Interational Astronomical Union defined the distance to be149,597,870,700 meters.

Historical Background: Tycho Brahe estimated the distance between the Sun

and the Earth at 8 million kilometers (5 million miles). Later, Johannes Kepler 

estimated the AU was at 24 million kilometers (15 million miles). In 1672, Giovanni

Cassini made a much better estimate by using Mars. By observing Mars from

Paris and having a colleague, Jean Richer, also observe Mars at the same time

in French Guiana in South America, Cassini determined the parallax of Mars.

From that Cassini was able to calculate the distance from Earth to Mars, and

then the distance from Earth to the Sun. Cassini calculated the AU to be at 140million kilometers (87 million miles), which is lower, but very close to the modern

day number.

2)  LIGHT YEAR

light-year is a unit of distance. It is the distance that light can travel in one year.

Light moves at a velocity of about 300,000 kilometers (km) each second. So in

one year, it can travel about 10 trillion km. More p recisely, one light-year is

equal to 9,500,000,000,000 kilometers.

Why would you want such a big unit of distance? Well, on Earth, a kilometer 

may be just fine. It is a few hundred kilometers from New York City to

Washington, DC; it is a few thousand kilometers from California to Maine. In

the universe, the kilometer is just too small to be useful. For example, the

distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion

km. That's 21,000,000,000,000,000,000 km. This is a number so large that it

becomes hard to write and hard to interpret. So astronomers use other units of

distance.

In our solar system, we tend to describe distances in terms of the AstronomicalUnit (AU). The AU is defined as the average distance between the Earth and the

Sun. It is approximately 150 million km (93 million miles). Mercury can be said to

be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the

Sun. The AU, however, is not big enough of a unit when we start talking about

distances to objects outside our solar system.

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For distances to other parts of the Milky Way Galaxy (or even further),

astronomers use units of the light-year or the parsec . The light-year we have

already defined. The parsec is equal to 3.3 light-years. Using the light-year, we

can say that :

  The Crab supernova remnant is about 4,000 l ight-years away.  The Milky Way Galaxy is about 150,000 light-years across.

  The Andromeda Galaxy is 2.3 million light-years away.

3) GALAXY

A galaxy is a massive, gravitationally bound system consisting of stars, stellar 

remnants, an interstellar medium of gas and dust, and, it is hypothesized, an

important but poorly understood component called dark matter .[1][2] The word

galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference

to the Milky Way. Examples of galaxies range from dwarfs with as few as ten

million (107) stars[3] to giants with a hundred trillion (1014) stars,[4] each orbiting

their galaxy's own center of mass. 

Galaxies contain varying numbers of star systems, star clusters and types

of interstellar clouds. In between these objects is a sparseinterstellar medium of

gas, dust, and cosmic rays. Observational data suggests that supermassive

black holes may exist at the center of many, if not all, galaxies. They are thought

to be the primary driver of active galactic nuclei found at the core of some

galaxies. The Milky Way galaxy appears to harbor at least one such object.

Where do White Dwarfs Come From?

Where a star  ends up at the end of its life depends on the mass it was born with.

Stars that have a lot of mass may end their lives as black holes or  neutron stars. A

low or medium mass star (with mass less than about 8 times the mass of our Sun)

will become a white dwarf. A typical white dwarf is about as massive as the Sun,

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yet only slightly bigger than the Earth. This makes white dwarfs one of the

densest forms of matter, surpassed only by neutron stars and black holes.

Black Hole  Neutron Star   White Dwarf 

Medium mass stars, like our Sun, l ive by fusing the hydrogen within their cores

into helium. This is what our Sun is doing now. The heat the Sun generates by its

nuclear  fusion of hydrogen into helium creates an outward pressure. In another 

5 billion years, the Sun will have used up all the hydrogen in its core.

This situation in a star is similar to a pressure cooker. Heating something in a

sealed container causes a build up in pressure. The same thing happens in the

Sun. Although the Sun may not strictly be a sealed container, gravity causes it to

act like one, pulling the star inward, while the pressure created by the hot gas in

the core pushes to get out. The balance between pressure and gravity is very

delicate.

When the Sun runs out of hydrogen to fuse, the balance tips in the favor of

gravity, and the star starts to collapse. But compacting a star causes it to heat

up again and it is able fuse what little hydrogen remains in a shell wrapped

around its core.

This burning shell of hydrogen expands the outer layers

of the star. When this happens, our Sun will become

a red giant; it will be so big that Mercury will be

completely swallowed!

When a star gets bigger, its heat spreads out, making

its overall temperature cooler. But the core

temperature of our red giant Sun increases until it's

finally hot enough to fuse the helium created fromhydrogen fusion. Eventually, it will transform the helium

into carbon and other heavier  elements. The Sun will

only spend one billion years as a red giant, as

opposed to the nearly 10 billion it spent busily burning

hydrogen.

(Betelgeuse) 

January 15, 1996,

Hubble SpaceTelescope Captures First

Direct Image of a Star,

A. Dupree (CfA) and

NASA. 

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We already know that medium mass stars, like our Sun, become red giants. But

what happens after that? Our red giant Sun will still be eating up helium and

cranking out carbon. But when it's finished its helium, it isn't quite hot enough to

be able to burn the carbon it created. What now?

Since our Sun won't be hot enough to ignite the carbon it its core, it willsuccumb to gravity again. When the core of the star contracts, it will cause a

release of energy that makes the envelope of the star expand. Now the star has

become an even bigger giant than before! Our Sun's radius will become larger 

than Earth'sorbit! 

The Sun will not be very stable at this point and will lose mass. This continues until

the star finally blows its outer layers off. The core of the star, however, remains

intact, and becomes a white dwarf. The white dwarf will be surrounded by an

expanding shell of gas in an object known as a planetary nebula. They are

called this because early observers thought they looked like the planets Uranusand Neptune. There are some planetary nebulae that can be viewed through a

backyard telescope. In about half of them, the central white dwarf can be seen

using a moderate sized telescope.

Planetary nebulae seem to mark the transition of a medium mass star from red

giant to white dwarf. Stars that are comparable in mass to our Sun will become

white dwarfs within 75,000 years of blowing off their envelopes. Eventually they,

like our Sun, will cool down, radiating heat into space and fading into black 

lumps of carbon. It may take 10 billion years, but our Sun will someday reach the

end of the line and quietly become a black dwarf. 

White dwarfs can tell us about the age of the Universe. If we can estimate the

time it takes for a white dwarf to cool into a black dwarf, that would give us a

lower limit on the age of the Universe and our  galaxy. But because it takes

billions of years for white dwarfs to cool, we don't think the universe is old

enough yet for many, if any, white dwarfs to have become black dwarfs.

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Finding black dwarfs would certainly alter our understanding of the cooling

process in white dwarfs

What Is a Black Hole? 

09.30.08 

An artist's drawing shows a large black hole pulling gas away from a nearby

star. Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet

View Larger Image → 

A black hole is a place in space where gravity pulls so much that even light

can not get out. The gravity is so strong because matter has been squeezed

into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible.

Space telescopes with special tools can help find black holes. The special tools

can see how stars that are very close to black holes act differently than other 

stars.

How Big Are Black Holes? 

Black holes can be big or small. Scientists think the smallest black holes are as

small as just one atom. These black holes are very tiny but have the mass of a

large mountain. Mass is the amount of matter, or "stuff," in an object.

Another kind of black hole is called "stellar." Its mass can be up to 20 timesmore than the mass of the sun. There may be many, many stellar mass black 

holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

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An artist's drawing shows the current view of the Milky Way galaxy. Scientific

evidence shows that in the middle of the Milky Way is a supermassive black 

hole. Image Credit: NASA/JPL-CaltechView Larger Image 

The largest black holes are called "supermassive." These black holes have

masses that are more than 1 million suns together. Scientists have found proof

that every large galaxy contains a supermassive black hole at its center. The

supermassive black hole at the center of the Milky Way galaxy is called

Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a

very large ball that could hold a few million Earths.

How Do Black Holes Form? 

Scientists think the smallest black holes formed when the universe began.

Stellar black holes are made when the center of a very big star falls in upon

itself, or collapses. When this happens, it causes a supernova. A supernova is

an exploding star that blasts part of the star into space.

Scientists think supermassive black holes were made at the same time as the

galaxy they are in.

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This image of the center of the Milky Way galaxy was taken by the Chandra X-

ray Observatory. Image Credit: NASA/CXC/MIT/F.K. Baganoff et al.

View Larger Image → 

If Black Holes Are "Black," How Do Scientists Know They Are There? 

A black hole can not be seen because strong gravity pulls all of the light into

the middle of the black hole. But scientists can see how the strong gravity

affects the stars and gas around the black hole. Scientists can study stars to

find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made.

This kind of light can not be seen with human eyes. Scientists use satellites and

telescopes in space to see the high-energy light.

Could a Black Hole Destroy Earth? 

Black holes do not go around in space eating stars, moons and planets. Earth

will not fall into a black hole because no black hole is close enough to the

solar system for Earth to do that.

This artist's drawing shows a supermassive black hole in the center of a galaxy.

The black hole is surrounded by a cloud of material that is spiraling into it.

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Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet

View Larger Image → 

Even if a black hole the same mass as the sun were to take the place of the sun,

Earth still would not fall in. The black hole would have the same gravity as the

sun. Earth and the other planets would orbit the black hole as they orbit the sun

now.

The sun will never turn into a black hole. The sun is not a big enough star to make

a black hole.

How Is NASA Studying Black Holes? 

NASA is using satellites and telescopes that are traveling in space to learn more

about black holes. These spacecraft help scientists answer questions about the

universe.

STARS

Young stars at this stage are called protostars. As they develop they accumulate massfrom the clouds around them and grow into what are known as main sequence stars. Mainsequence stars like our own sun exist in a state of nuclear fusion during which they will emitenergy for billions of years by converting hydrogen to helium.

Stars evolve over billions of years. When their main sequence phase ends they passthrough other states of existence according to their size and other characteristics. The

larger a star's mass, the shorter its lifespan will be.

 As stars move toward the end of their lives much of their hydrogen has been converted tohelium. Helium sinks to the star's core and raises the star's temperature—causing its outer shell to expand. These large, swelling stars are known as red giants.

The red giant phase is actually a prelude to a star shedding its outer layers and becoming asmall, dense body called a white dwarf. White dwarfs cool for billions of years, until theyeventually go dark and produce no energy. At this point, which scientists have yet toobserve, such stars become known as black dwarfs.

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